This invention relates to high strength, low alloy steel products and methods of producing such products, the invention being more particularly related to hot rolled steel products of the stated character with improved mechanical properties, particularly in formability as by shaping, bending and like operations involving drawing strain in the metal or portions of it. Certain conventional HSLA steels, e.g. produced as hot rolled strip, have successfully included one or more micro-alloying ingredients such as columbium, vanadium and titanium, such steel being fully killed, i.e. usually aluminum-killed, and the strip or like article produced for use in as-hot-rolled condition having a yield strength in the range of about 50 ksi and above, indeed often as high as 80 or 90 ksi. In order to avoid directionality of results in forming processes such as bending and in impact strength, e.g. relatively poor transverse properties, so-called sulfide shape control agents have often been added, usually rare earth elements.
Although hot rolled, aluminum-killed products of the foregoing sort, sometimes with very low carbon (e.g. 0.06% or less) and including columbium or both columbium and vanadium for tensile properties and toughness, have been successful, it has been found that some difficulties nevertheless remain. For example, these hot rolled products have exhibited some limitations upon forming, notably bending, in that even though rare earths are added to minimize directionality or special chemistry is adopted for like purpose, cracking sometimes occurs, e.g. edge or other surface cracks, with small radius bends that are hoped to be possible at low carbon levels of steel.
In the case of some rimming grades of steel, particularly as designed for deep drawing operations in cold rolled state, i.e. after extensive cold rolling with appropriate annealing, problems of an apparently different sort have been noted and sought to be overcome. This rimming steel, which customarily is made with 0.07 to 0.11% carbon for convenience and economy in production, has superior drawing properties with a clean surface essential for cold rolled strip to be deep drawn or similarly formed, but manipulating or bending the strip or otherwise subjecting it to drawing or deforming operations has a tendency to fluting, and the steel has also exhibited the surface defects or markings known as stretcher strains. Other problems have been that the rimmed steel has some undesirable internal porosity and is subject to aging, i.e. so-called strain aging, which can also result in the stretcher strain markings mentioned above.
With the view of avoiding various difficulties in cold rolled drawing grades derived from rimmed steel, it has been proposed to prepare ingots using a rimming steel composition as described above, by first pouring each ingot mold to about 80% to 90% full of the molten steel, then interrupting such pour and allowing the mold to rest while rimming action occurs, e.g. for several minutes, and a skin or shell of rimmed steel solidifies adjacent to the mold wall. Thereupon pouring is continued, i.e. of the same steel from the same ladle, to fill the mold, while aluminum is added in sufficient amount to kill the entire, still-molten steel core. When the ingot solidifies, it consists of a rimmed steel skin or layer around and integral with a killed steel core, and can be processed by the usual steps of hot rolling, cold rolling, annealing and temper rolling as appropriate to achieve a cold rolled product of strip or the like suitable for drawing applications and retaining the rimmed surface over aluminum-killed steel. It is said that such products avoid the aging and porosity problems of previous rimmed steel, and may avoid the difficulties of the latter as to fluting and stretcher strains. In some such proposals for cold rolled, deep drawing products, addition of special elements such as columbium (for hardening or strengthening) and rare earths (for sulfide shape control) to the molten core have been described. In another case, it was proposed to fill the mold to about 65% with rimming (effervescing) steel of 0.08% C and 0.38% Mn, and after solidification of a shell, to complete filling with a killed steel, high carbon melt of 0.78% C, 0.80% Mn and 0.26% Si, said to yield a deep drawing product of high tensile strength, free of metalloid segregation.
None of this prior art, however, has indicated any significance for the class of steel now contemplated as high strength low alloy (HSLA) material with relatively quite low carbon content and designed for employment as a hot rolled product, e.g. hot rolled strip having sufficient levels of yield and impact strength, and bending properties, to be attractive for automotive uses where high strength with less weight has become important. Indeed, these HSLA steels inherently lack the problems, such as aging, porosity, or stretcher strains that have been noted with cold rolled, drawing grades of rimmed steel, but the above developments in the art have been noted because of their possible superficial resemblance, in some procedural steps, to some operations that are involved in the present invention and that have been discovered to have unexpected benefit for the special class of as-hot-rolled HSLA steel products.